Automated fiber placement devices form composite parts by applying a plurality of narrow strips of material (e.g., carbon fiber impregnated with resin) referred to as tows. An automated fiber placement machine may apply tows by depositing the tows with a roller or other device. For example, the tows may be compacted onto a layup surface by one or more compaction rollers. The automated fiber placement device applies or places multiple adjacent tows to form a contiguous array or band, referred to as a course. Each tow of the course has a fixed width. Multiple courses are applied to form a single layer of tows, referred to as a ply. A composite part includes multiple plies (e.g., a set of plies), and plies of the set of plies are often oriented in different directions (e.g., a 0 degree ply, a +45 degree ply, a 90 degree ply, and a −45 degree ply). An orientation of the ply refers to a nominal direction of the tows (and the composite fibers thereof) of the ply.
During tow application, some automated fiber placement devices can steer the tows of a course to follow a non-linear path (e.g., a curved path). If a course does not follow a geodesic path (e.g., a shortest straight line path on a surface) the course is said to be steered at a steering radius. The steering radius is the reciprocal of the geodesic curvature, usually measured at a particular point along a course centerline. While steering a course, the distance traveled by an inner edge of a roller is less than the distance traveled by an outer edge of the roller. Because the tows are dispensed at a constant rate across the width of the roller, excess composite tow material builds up in front of the roller when the tows of the course are steered. The excess tow material can cause wrinkling (e.g., an edge of the tow folds over on the tow) and puckering (e.g., a buckling of the tow) of tow material to occur. Typically, an upper limit on allowed steering radius is used to avoid wrinkling or puckering. However, using the steering radius (e.g., a local measurement) to predict and prevent tow wrinkling or puckering does not take into account any cumulative steering effects along a course.
Wrinkling or puckering of tows may cause local distortions of the fibers, which may negatively affect the fiber's capability, and consequently the composite part's capability, to carry load. A puckered tow might also fold over when a course of a subsequent ply is laid down, causing an inhomogeneous distribution of the fibers in the ply, as well as a thickness variation. An inhomogeneous distribution of the fibers may cause a local weakness in the part, affecting its structural capability. A thickness variation may affect the final surface quality of the part, which may require rework through filling, sanding, or shimming.
Wrinkled or puckered tows may be reworked by hand by lifting and repositioning tows before any subsequent plies are laid down. Puckers may temporarily be suppressed by locally compacting the area near the pucker by hand, or by compacting the entire part under vacuum. Reworking the part adds time and cost to the production of the part. If the part design is such that the desired fiber orientation requires a tow to follow a particular path, reapplying a tow by hand may not eliminate the pucker or wrinkle, and any reduction in load carrying capability of the part due to wrinkles may need to be mitigated by adding material, resulting in a heavier part.